Remote emission sensing (RES) systems are known. One such system is disclosed in U.S. Pat. No. 5,210,702 and comprises an electromagnetic (EM) radiation source that is arranged to pass a beam of EM radiation through the exhaust plume of a motor vehicle as the motor vehicle passes by the system. The system also comprises one or more detectors arranged to receive the radiation after it passes through the exhaust plume of the vehicle. One or more filters may be associated with the one or more detectors to enable the detectors to determine the intensity of EM radiation having a particular wavelength or range of wavelengths. The wavelengths may be conveniently selected to correspond to wavelengths absorbed by molecular species of interest in an exhaust plume (e.g., hydrocarbons (HC), carbon monoxide (CO), carbon dioxide (CO2) and nitrogen oxides (NOx) such as NO and NO2. The one or more detector output voltages represent the intensity of the EM radiation measured by that detector.
These voltages are then input to a processor. The processor calculates the difference between the known intensity of the light source and the intensity detected by the detectors to determine the amount of absorption by the particular molecular species (based on predetermined wavelengths associated with that species). Based on the measured absorption(s), the concentration of one or more molecular species in the emissions may be determined in a known manner.
A system for the remote sensing of exhaust opacity is disclosed in “Feasibility of Remote Sensing of Particulate Emissions From Heavy-Duty Vehicles,” Chen, G. et al., American Society of Automotive Engineers (1996). In this system, opacity is measured at a wavelength of 638 nm and correlated with CO2 measurements.
Existing RES systems suffer from various drawbacks and limitations. These factors may lead to erroneous readings, a relatively high incidence of discarded data or a relatively high incidence of “flagged” test results. These and other problems can reduce the benefits of an RES system.
At least some RES systems work, in part, by determining the absorption (or transmittance) of light through an exhaust plume. By determining the absorption/trasmittance at particular wavelengths (corresponding to wavelengths at which various molecular species present in an exhaust plume absorb EM radiation), the concentration of those species in the exhaust can be determined. One problem is that various outside factors may affect the measured intensity and lead to errors. For example, if the measured intensity is reduced due to light scattering by particles in the exhaust plume, rather than absorption of the radiation by the species of interest, this can lead to errors.
One drawback of some remote sensing systems is the use of a single wavelength of EM radiation to measure opacity. It is known that EM radiation scattering due to the presence of particles increases with decreasing wavelength. Because scattering is a major contributor to a plume's opacity, systems that measure opacity with only relatively long wavelength EM radiation may often yield inaccurate results.
These and other drawbacks exist.